An antibody is composed of four polypeptides: two heavy chains and two light chains. The antigen binding portion of an antibody is formed by the light chain variable domain (VL) and the heavy chain variable domain (VH). At one extremity of these domains six loops form the antigen binding site and also referred to as the complementarity determining regions (CDR). Three CDRs are located on the VH domain (H1, H2 and H3) and the three others are on the VL domain (L1, L2 and L3). During B cell development a unique immunoglobulin region is formed by somatic recombination known as V(D)J recombination. The variable region of the immunoglobulin heavy or light chain is encoded by different gene segments. The heavy chain is encoded by three segments called variable (V), diversity (D) and joining (J) segments whereas the light chain variable is formed by the recombination of only two segments V and J. A large number of antibody paratopes can be generated by recombination between one of the multiple copies of the V, D and J segments that are present in the genome. The V segment encodes the CDR1 and CDR2 whereas the CDR3 is generated by the recombination events. During the course of the immune response further diversity is introduced into the antigen binding site by a process called somatic hypermutation (SHM). During this process point mutations are introduced in the variable genes of the heavy and light chains and in particular into the regions encoding the CDRs. This additional variability allows for the selection and expansion of B cells expressing antibody variants with improved affinity for their cognate antigen.
The vast majority of immunoglobulins are bivalent and monospecific molecules carrying the same specificity on both arms as they are composed of two identical heavy chain polypeptides and two identical light chain polypeptides. However, it was recognized very early during the development of hybridoma technology that hybrid hybridomas can be created by a fusion event between two hybridomas (Suresh M R et al., Methods Enzymol 1986; 121: 210-228). These ‘quadromas’ express two different heavy and two different light chains and therefore produce a variety of different antibody species resulting from the random pairing of the heavy and light chains. Amongst these different species, bispecific antibodies (bsAbs) are generated, carrying a different specificity on each arm. Another naturally occurring exception is the immunoglobulin of the IgG4 isotype that is able to undergo heavy chain exchange due to a less stable dimerization mediated by the hinge region of that isotype (van der Neut Kolfschoten M et al., Science. 2007 317(5844):1554-7). Although this exchange seems to happen in vivo, its biological significance remains unclear.
Monoclonal antibodies have emerged as a successful and attractive class of molecules for therapeutic intervention in several areas of human disease. However, targeting or neutralizing a single protein is not always sufficient to achieve efficacy in certain diseases which limits the therapeutic use of monoclonal antibodies. It is increasingly clear that in a number of indications neutralizing one component of a biological system is not sufficient to achieve efficacy. One solution to this problem is the co-administration of several monoclonal antibodies. This approach is however complicated by regulatory aspects if the antibodies to be combined have not been previously approved individually. Moreover, combination approaches are also costly from a manufacturing perspective. Accordingly, there exists a need for antibodies and therapeutics that enable targeting of multiple antigens with a single molecule.